The advent of prepreg technology for high-performance aerospace, railway, energy, marine, sports and leisure applications has resulted in the pioneering use of multi-layered composites, or sandwiches, for structural applications.
A prepreg consists of a combination of a matrix (resin) and fiber reinforcement in the form of woven or non-woven mats. Fiber reinforcement is available in unidirectional form (one direction of reinforcement) or in fabric form (several directions of reinforcement). The role of the matrix is to support the fibers and bond them together in the composite material. In order to give the finished composite environmental and temperature resistance, matrix resins are in their overwhelming majority thermoset materials.
Depending upon final temperature exposure and desired mechanical properties, the resin is selected from three different polymer classes: phenolic, epoxy and bismaleimide/polyimide. Recent developments, however, have demonstrated that unsaturated polyester resins, which are easier to handle than epoxy resins, can be also used for prepreg manufacturing.
Wetted fibers, through the application of the matrix resin, become stronger and stiffer; as a result, they support most of the applied loads. Unidirectional composites have mechanical properties in one direction (anisotropic); on the other hand metals are isotropic, i.e. have equal properties in all directions.
Prepreg sandwich constructions (FIG. 1) are the most prevalent composites in use today. Such constructions consist of thin, high strength skins (or outer layers) bonded through the optional use of an adhesive film layer to a thicker, light weight core material that can be balsa wood, foam or a circular or polygonal-hollowed construction, generally a honeycomb having an hexagonal shape (FIG. 2).
Performance characteristics of this type of construction are:
Tensile and compression stresses are supported by the skins,
The skins are stable across their whole length,
Shearing stress is supported by the core (honeycomb),
Rigidity is available in several directions, and
Excellent weight savings.
Prepreg sandwiches can be manufactured by complex and costly processes such as autoclaving or vacuum bag molding. However, press or compression molding has also been used (FIG. 3) and has become more prevalent.
In summary, prepreg technology is a complex and costly process whose use is justified by the demanding and highly critical technical requirements of targeted aerospace applications. Its benefits are lower weights than these available with previously-used metallic constructions, greater strength and higher stiffness.
More recently, technical advances in the field of polyurethane chemistry and processing have made the prepreg sandwich concept and its production more affordable and easier to implement. This has resulted in molded polyurethane articles, such as polyurethane foams and polyurethane composites that are now extensively used by the automotive, construction and furniture industries. Such articles are generally available in the form of sandwich, or multi-layered, constructions, that produce high strength, structural parts.
The leading polyurethane technology, marketed under the trade name Baypre® F (Bayer AG, Germany), is described in U.S. Pat. No. 6,156,811 (hereafter referred to as the '811 patent) and U.S. Pat. No. 7,419,713 B2 (hereafter referred to as the '713 patent).
This approach relies on a multi-step process whereby a composite component is created by producing a sandwich structure made of at least one core layer located between at least two outer layers. The steps described in the '713 patent are the following (FIG. 4):
(i) Insert the core and the outer layers into a compression mold, the core being located between the outer layers. The core is a honeycomb structure and the outer layers are glass fiber or natural fiber mats.
(ii) Apply a polyurethane resin either by a casting or a spraying operation on one or both outer layers. Steps (i) and (ii) can be performed in any order.
(iii) Compression mold under high heat the core with the outer layers to cure the resin and form the sandwich structure.
(iv) Optionally attach decorative layers on top of the top outer layer and or below the bottom outer layer only after steps (i) to (iii) are completed.
The present invention improves on the above process by sequentially placing bottom outer layer, core layer, and top outer layer in the compression mold and allowing the finished part or component to be manufactured in a single press mold, using a single step or molding process, without the additional step of applying a polyurethane resin. Optional bottom decorative layers and top decorative layers can be added as part of the process, if desired.
The present invention therefore presents two key benefits over the prior art, by providing a single step process to produce a finished component, including the optional addition of decorative layers, in a single mold, and the elimination of resin application to the outer layers which results in less expensive and lighter weight panels.